The invention relates to a composite system comprising at least one biodegradable blend, containing at least one biodegradable block copolyester urethane (PEU) and also at least one polyhydroxyalkanoate (PHA), possibly a filler comprising a polysaccharide and/or derivatives thereof and also possibly further biocompatible additives. Composite systems of this type are used for producing moulded articles, moulded parts or extrudates. Furthermore, the invention relates to possibilities for using the composite system.
Poly-(R)-3-hydroxyalkanoates (PHA), such as e.g. poly-(R)-3-hydroxybutyrate (R-PHB), are an almost ideal polymer material from an environmental point of view and from the aspect of sustainability. They are produced from waste of sugar production, i.e. from renewable raw materials, by means of bacterial fermentation on an industrial scale. It is stable under conditions in which plastic materials are normally used but can be biodegraded within weeks to months in a dump or in a composting process. R-PHB can be processed thermoplastically and can be readily recycled as a thermoplastic. It is biocompatible, biodegradable and can be used as a component of implant materials and as a good substrate for cell growth. By means of degradation of R-PHB, stereoregular organic synthesis structural units could be obtained.
The R-PHB obtained from bacteria has however unfavourable material properties for many applications. It is brittle and inelastic and the production of transparent films is not possible. The melting point at 177° C. is so high that only a relatively small temperature range for thermoplastic processing results until the start of decomposition at approx. 210°. All these disadvantages result from the high crystallinity of the R-PHB. Finally, also cell fragments which decompose during processing often remain from the reprocessing of the biological material, which leads to an unpleasant odour problem.
In order to eliminate the difficulties of the thermoplastic processing, two routes have been adopted above all. It has been attempted therefore, on the one hand, to set low processing temperatures by means of physical measures, in particular by delaying crystallisation. On the other hand, bacterial cultures and substrates which enable the production of copolymers, in particular poly-3-hydroxybutyrate-co-3-hydroxyvalerate, have been used. In the first case, ageing leads nevertheless to recrystallisation, i.e. embrittlement. In the latter case, a reduction in the melting temperature and an increase in the elasticity is indeed achieved, however the possibility of controlling the properties by bacterial copolymerisation is provided only within narrow limits.
Poly-(R)-3-hydroxybutyrate-covalerate (PHB-HV) is in fact lower-melting; however bacteria incorporate only a relatively small hydroxyvalerate proportion (up to approx. 12%), whilst good material properties are anticipated only after above approx. 20% HV proportion.